Field emission displays (FEDs) have been known for a number of years as an alternative to active matrix liquid crystal displays (AMLCDs). Indeed FEDs are known to have a number of advantages. They can be made thinner and lighter while having better operating characteristics compared to an AMLCD. In particular an FED may have a higher contrast ratio, wider viewing angle and greater brightness. They can operate on lower power and over a wider range of operating temperatures.
The major drawback preventing wider use of FEDs, however, is their cost. They are expensive to manufacture, for reasons which become apparent when you examine more closely their structure.
FIG. 1 shows in cross-section a conventional field emitter arrangement, a number of which may be arranged in an array to form a display. The emitters 1 comprise cones of metal deposited onto a cathode 2. The emitters 1 are located between gate electrodes 3 that themselves are mounted on the cathode 2 through insulating material 4. When a voltage is applied between the gate and emitter electrodes electrons are emitted from the tips of the emitters 1 and these electrons strike a phosphor coated faceplate where they create illumination through the conventional process of electroluminescence.
A single visible pixel in a display may contain as many as 1600 such field emitters and therefore it will be appreciated that the dimensions involved are small. The spacing between the tip of a field emitter and the gate electrodes, for example, is of the order of 0.5 micron, and it is critical in determining parameters such as turn-on voltage and current density. In addition, to provide strong electron emission, the field emitter tip should be as sharp as possible. These requirements place stringent conditions on the manufacturing process and while effective field emission displays using the design of FIG. 1 can be manufactured, they are very expensive and their use is limited as a consequence.
There is therefore a need to provide a field emission display design that would be simpler (and therefore cheaper) to manufacture. One possible design has been proposed in H. Busta et al., "Volcano-Shaped Field Emitters for Large Area Displays", Proc. IEDM, pp 405-408, 1995. FIG. 2 illustrates schematically the type of field emitter proposed in this paper. Effectively the traditional design is inverted and instead of an emitter being placed between two gate electrodes, an emitter is placed on either side of a post gate.
In FIG. 2 a single gate post 5 is formed on a layer of n-silicon. This gate 5 is raised from the surface of the n-silicon substrate and is generally circular or regular polygonal in cross-section (hence the description "volcano shaped"). On this substrate there is deposited firstly an insulating layer of silicon dioxide 6 and then an emitter layer 7. This manufacturing process is considerably simpler and the dimensions less critical than in the conventional FED manufacturing process. Unfortunately, however, the resulting FED does not have high performance characteristics for a number of reasons.
Firstly, as can be seen from FIG. 2, the ends 7' of the emitter layer adjacent to the gate post--and from which electrons will be emitted--do not form a point which fact results in reduced electron emission. Secondly, one of the advantages of a conventional design as shown in FIG. 1 is that the field emitter is located symmetrically between two gate electrodes. This means that when an electron is emitted there is no tendency for it to be drawn to one gate electrode or the other and it passes between them towards the phosphor face plate. However, in the design proposed by Busta et al this symmetry is lost and a significant number of electrons emitted by the emitter are drawn to the gate post to form a leakage gate current.
The design proposed by Busta et al provides good uniformity over large areas without the need for expensive sub-micron manufacturing techniques. However the turn-on voltage is increased substantially. Furthermore due to the circular nature of the shape of the emitter a low cell density results. This in turn causes the field emission current density to be small. In addition this structure gives a large beam spot which is not always desirable. These and other disadvantages mean that the performance of a field emitter according to the proposal of Busta et al is disappointing.